THE LEAF 



211 



well developed, but are distinctly degenerate in the higher subtribes 

 of the group and are less well developed in any living conifers than 

 they are in Prepinus and Cretaceous species of Finns. The 

 Gnetales often show the transfusion tissues in a high degree of 

 development. The small leaves of Ephedra naturally show them 

 less distinctly than the large persistent foliar organs of Welwitschia, 

 where they constitute a very conspicuous feature of the organiza- 

 tion of the leaf. In Gnetum, again, in accordance with its higher 



FIG. 158. Base of leaf of Casuarina eqitisetifolia, showing transfusion tissue 



systematic position, the transfusion sheath is less conspicuous. 

 Among the angiosperms transfusion tissues are present in the 

 dicotyledons, but present themselves in the condition typical for 

 the higher gymnosperms only in the genus Casuarina. Here, as is 

 shown in Fig. 157 illustrating the organization of a leafy twig of 

 Casuarina equisetifolia, there are clusters of thick-walled empty 

 cells flanking the leaf traces. This relation to the foliar strands 

 strikingly resembles that found in the higher gymnosperms and 

 appears to be good evidence of the primitive position of this interest- 

 ing genus. In Fig. 158 is shown one of the foliar bundles of the 



